Method and apparatus for heat treatment



y 1969 J. DEANS 3,456,936

METHOD AND APPARATUS FOR HEAT TREATMENT Filed Aug. 22, 19s? IN VENTOR. JACKSON DEANS w e AGENT United States Patent US. Cl. 26347 7 Claims ABSTRACT OF THE DISCLOSURE A method of heat treating material in a vapor atmosphere wherein the material to be treated is placed within a closed oven connected at one end to a tube which contains a substance in liquid form providing when heated the desired vapor pressure in the atmosphere. The oven and the tube are heated in such a manner as to establish the coldest spot adjacent the tube and a decreasing temperature gradient along the tube, and a desired back pressure is established at the remote side of the liquid substance which plugs up the tube. As vapor pressure variations occur within the oven atmosphere, the liquid-vapor interface gets displaced to a tube location at a different temperature which changes the vapor pressure to compensate for its variations.

This invention relates to methods of heat-treatment.

When a mass of material is heat-treated in an oven containing the vapour of a substance under pressure, difficulties may arise, inter alia the pressure may increase rapidly so that the oven fractures.

The invention is based on the recognition that the vapour pressure in a heated space is determined by the coolest part of the space.

According to the invention, in a method of heat-treating a mass of material in an oven containing the vapour of a substance under pressure, the oven space is connected to a pressure source by way of a tube containing a liquid plug of the substance and the heating is such that the part of the oven space adjacent the plug is in the coolest part of the oven space and there is a fall in temperature or decreasing gradient along the tube from the part of the oven space adjacent the plug in the direction away from the oven space whereby with increase in pressure in the oven space the liquid/vapour interface of the substance moves to a cooler part of the tube and with a decrease in pressure in the oven space the interface moves to a hotter part of the tube. The method according to the invention may readily be made self-regulating against the random temperature and pressure variations which normally occur without the provision of a temperature control device operating to a very high degree of accuracy.

The mass of material may be in divided form or in the form of a coherent body, for example in single crystal form. After treatment, divided material may remain divided or, having been melted during the treatment, may be in the form of a coherent body. If a seed crystal is provided, an initially divided mass or an initially polycrystalline coherent body may solidify after melting in single crystal form.

It may be advantageous if pressure is exerted on the plug by a gas inert with respect to the liquid substance and acting on the side of the plug more remote from the oven space along the tube.

The plug may be provided in a part of the tube comprising a section of narrow bore. The section of narrow bore provides a damping action on the movement of the plug, which may be desirable if it is possible that a large alteration of conditions in the oven space occurs and may also assist in maintaining the desired temperature gradient along the tube in that flow of heat along the tube and through the plug away from the oven space is limited to a desired value.

The heat-treatment may be a zone melting treatment in which the vapour is absorbed in the material. The material may be of germanium, either initially intrinsic or initially extrinsic, and the substance may be mercury.

The method according to the invention has been found to be a satisfactory alternative to the method described in J. App. Phys. vol. 33, No. 10 October 1962, pp. 2947 to 2950 in which it is stated that germanium with concentrations between 8x10 and 5x10 atoms of mercury per cc. have been obtained in an open system not under pressure: Proc. National Electronics Conference vol. 19 (1963) Paper 1656, pp. 635 to 645 describes a method by which higher concentrations of 0.5 X10 to l 10 atoms of mercury per cc. are obtained, using a sealed ampoule to provide the pressure necessary for obtaining the higher doping range which may be desired for some uses.

For a zone-melting heat treatment, the oven space may be heated generally with the aid of a first system and local heating for melting a zone of the material may be provided by a second heating system which is moved relative to the oven space.

For germanium and mercury vapour, the oven may be so heated that in the steady state the minimum temperature in the oven space is between 450 C. and 500 C. It has been found that with a minimum temperature in the oven space of 480 C. a doping level for germanium of 2x10 atoms of mercury per cc. can be achieved.

The invention also relates to a mass of material when heat-treated by the method according to the first aspect of the invention.

An embodiment of the method according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing which shows an apparatus for carrying out the method according to the invention.

The figure shows a crucible 1 of high purity silica and of semi-cylindrical form containing polycrystalline germanium 2 and a seed crystal 3.

The crucible is arranged in an elongated silica tube 4 sealed at one end 5 and at the other end 6 connected to a silica tube 7 of smaller cross-section and comprising a section 8 of narrow bore. The section 8 is coiled and may be about 1 ft. in length, measured along the tube and from /2 mm. to 1 mm. in internal cross-section diameter. The tube 7 includes a reservoir 9 and contains a plug of mercury 10.

The tube 4 is inclined downwards towards the end 6 and has an internal bafile 11 against which is provided a pile 12 of broken silica.

The oven space, that is the space Within the tube 4 and the communicating space within the tube 7 as far as the mercury 10, is heated generally with the aid of a first heating system comprising a resistive heating part which may be wound on the tube 4 and two muflle parts 14 to give end correction so that the oven space is heated substantially uniformly but with the part of the oven space adjacent the mercury 10 as the coolest part of the oven space.

Local heating for zone melting the germanium 2 is provided by a second heating system which is movable and comprises a resistive heating part 15 which may be a winding provided on a hollow cylindrical former.

The sequence of operations will now be described.

With the end 5 of the tube 4 open, the silica pile 12 3 and the crucible I loaded with the germanium 2 and the seed 3 are provided in the tube 4. The end 5 of the tube 4 is then partially closed to facilitate later sealing.

The end 6 of the tube 4 is joined to the tube 7 which beyond the reservoir 9 is connected to a branched tube one arm 18 of which is connected by way of a cock 19 to a rotary backing pump used in an evacuation system (not shown) and the other arm 20 of which is connected by way of a cock 21 to a source of nitrogen under pressure (not shown). A pressure gauge 22 is connected to the branched tube at a position between the cocks 19 and 21.

An aperture is made in the tube 4 at 16, mercury is introduced into the tube 4 through the aperture at 16 which is then rescaled. The mercury is retained below the bafile 11 in the lower end of the tube 4.

The end 5 of the tube 4 is connected to a diffusion pump contained in the evacuation system comprising the backing pump connected to the arm 18. The tube 4 is evacuated, for example, to a pressure of 2/ of mercury, and the end 5 is sealed.

The mufile parts 14 are adjusted in position as is the movable part 15 and the heating systems are switched on to raise the temperature of the oven space generally to about 600 C. and to form a zone of molten germanium adjacent the seed crystal 3.

Cock 19 is closed and pressure is applied slowly from the nitrogen source by adjustment of cock 21. It is mentioned that time must be allowed for the building up of back pressure by evaporation of mercury 10.

The germanium 2 is then zone heat-treated by moving the part 15 and hence the molten zone to the left hand side of the figure. The resolidified germanium is single crystal material, seeded by the seed 3, and is doped with mercury, from the ambient mercury-containing atmosphere.

Cock 21 is closed and cock 19 is partially opened so that nitrogen is slowly pumped away by the backing pump. Finally, cock 19 is fully opened and hence the mercury 10 is withdrawn to the reservoir 9.

The heating systems are switched off and the germanium 2 is allowed to cool. Cock 21 is closed, the end 5 of the tube 4 is opened and the crucible 1 is unloaded.

During the zone-heating operation random variation may increase the pressure in the oven space. This has the effect of pushing the mercury 10 away from the end 6 and the pressure stabilizes when the mercury vapour pressure drops as the mercury 10 is removed to add to the oven space a cooler part of the tube 7. The opposite effect occurs with random decrease in the pressure in the oven space. The provision of the section 8 tends to damp too rapid movement of the mercury 10 and also has an effect in determining the temperature drop along the tube 7.

At the equilibrium pressure selected, determined by the height of the mercury 10 and the pressure applied from the nitrogen source, mercury dissolved in the melt until the condition is reached in which the vapour pressure over the solution corresponds to the applied pressure so that the mercury in the treated germanium is dependent 0n the pressure applied and also the segregation coefficient of mercury in germanium. With the oven space temperature generally at about 600 C., the minimum temperature in the oven space may readily be made between 450 C. and 500 C.

What is claimed is:

1. A method of heat-treating a mass of a material in the closed space within an oven containing the vapor of a substance under pressure derived from a liquid plug of the substance provided in a tube connected to the oven space, comprising providing within the tube the said substance so as to fill its cross-section forming a liquid plug having at one side within the tube and facing the oven space an interface with the substance vapor, heating the oven space in such manner that the part of the oven space adjacent the plug is the coolest part and such that there is established along the tube containing the liquid-vapor interface a temperature gradient which decreases in a direction away from the oven space, and establishing a pressure at the opposite side of the liquid plug, whereby variations in the oven space of the vapor pressure of the substance displaces the liquid-vapor interface to a tube location at a dilferent temperature to compensate for the pressure variation.

2. A method as set forth in claim 1 wherein a fixed pressure is established at the opposite side of the plug by bringing a gas inert with respect to the substance at a controlled pressure into contact with said opposite side of the plug.

3. A method as set forth in claim 1 wherein the material is germanium and the substance is mercury, and the germanium is subjected to a zone-melting treatment within the oven space in an atmosphere of mercury vapor to incorporate mercury atoms in the germanium.

4. A method as set forth in claim 3 wherein the heating is such that the coolest part of the oven space in the steady state is between 450 and 500 C.

5. Apparatus for heat-treating a mass of material in the vapor of a substance under pressure, comprising a closed oven connected at one end to one end of an elongated tube of smaller size than that of the oven, a vessel connected to the opposite end of the tube, a supply of said substance in the liquid state filling a part of the tube and a part of the vessel, means for heating the oven and the tube in such manner that the coolest part of the oven is adjacent the tube and a decreasing temperature gradient is established along the tube in a direction away from the oven and toward the vessel, and means connected to the vessed to establish a controlled pressure therein over the surface of the liquid substance.

6. Apparatus as set forth in claim 5 and including means for heating and melting a zone of the mass of material and means for moving the melted zone through the mass.

7. Apparatus as set forth in claim 5 wherein the tube comprises a coiled section having a narrow bore.

References Cited UNITED STATES PATENTS 3,003,900 10/1961 Levi 148-189 3,183,130 5/1965 Reynolds et al. 263-47 3,183,131. 5/1965 Huffman 148-189 3,275,557 9/1966 Hughes 148-189 X JOHN J. CAMBY, Primary Examiner U.S. Cl. X.R. 

